For many diseases, such as infectious diseases, vaccines have been developed or are in the process of being developed. These vaccines induce a protective immune response within a certain time frame. Since most vaccines are used for the vaccination against diseases that are rather rare in the population, there is usually no need that the generation of the immune response be particularly rapid. However, there are situations in which an immune response, such as a protective immune response, should be generated as fast as possible. This may be the case in an outbreak of smallpox or in any other human poxvirus disease.
The causative agent of smallpox is the variola virus, a member of the genus Orthopoxvirus. Vaccinia virus, also a member of the genus Orthopoxvirus in the family of Poxviridae, was used as a live vaccine to immunize against smallpox. Successful worldwide vaccination with Vaccinia virus culminated in the eradication of variola virus (The global eradication of smallpox. Final report of the global commission for the certification of smallpox eradication; History of Public Health, No. 4, Geneva: World Health Organization, 1980). In the meantime, most of the stocks of infectious variola viruses have been destroyed. However, it can not be excluded that poxviruses, inducing smallpox or smallpox-like diseases, might again become a major health problem. In addition, there is a risk that a poxvirus disease of animals is spread to humans.
Moreover, there may also be other situations in which it is desirable to induce a rapid immune response. For example, it might be desirable to induce a rapid immune response against diseases that are endemic in some parts of the world, if it is necessary to travel to such a country at short notice.